Formation of difluoromethylene - arenium complexes by consecutive aryl - CF3 C - C bond activation and C - F bond cleavage

Article abstract of DOI:10.1021/ja990779g

Oxidative addition of one of the strongest C - C bonds, Aryl - CF₃, to Rh(I) takes place upon treating 1-CF₃-2,6-(CH₂P^tBu₂) ₂-C₆H₃ (1, PCP) with [RhClL₂]

Full text of DOI:10.1021/ja990779g

6652
J. Am. Chem. Soc. 1999, 121, 6652-6656
Formation of Difluoromethylene-Arenium Complexes by
Consecutive Aryl-CF3 C-C Bond Activation and C-F Bond
Cleavage
Milko E. van der Boom, Yehoshoa Ben-David, and David Milstein*
Contribution from the Department of Organic Chemistry, The Weizmann Institute of Science,
RehoVot 76100, Israel
ReceiVed March 9, 1999
Abstract: Oxidative addition of one of the strongest C-C bonds, Aryl-CF3, to Rh(I) takes place upon treating
1-CF3-2,6-(CH2PtBu2)2-C6H3 (1, PCP) with [RhClL2]2 (L ) C2H4 or C8H14) in dioxane or toluene at elevated
temperatures leading to quantitative formation of Rh(CF3)(2,6-(CH2PtBu2)2-C6H3)Cl (2-Cl). The iodide analogue
2-I was prepared by reacting Rh(η1-N2)(2,6-(CH2PtBu2)2-C6H3) (3) with CF3I at room temperature. ArCF2-F
bond cleavage was not observed in parallel to the C-C bond activation. Treating a dioxane solution of the
thermally stable RhIII-CF3 complexes 2-Cl,I with excess trifluoromethanesulfonic acid (HOTf) at room
temperature resulted in CsF bond cleavage and selective formation of the unique difluoromethylene-arenium
complexes [Rh(1-CF2-2,6-(CH2PtBu2)2-C6H3)X][OTf] (4; X ) Cl, I) which were characterized spectroscopically
by NMR, UV/vis, and FD-MS. No reaction was observed with HCl. Reaction of 2-Cl with BF3 or Ph3CBF4
(trityl cation) also resulted in C-F bond cleavage to give [Rh(1-CF2-2,6-(CH2PtBu2)2-C6H3)Cl]BF4 (10).
Introduction
by reaction of the resulting RhIII-CF3 complex with protonic
or Lewis acids to form unique difluoromethylene-arenium
complexes by C-F cleavage and C-C bond formation. Part of
this study has been communicated.3i Arenium or Wheland
complexes are transient intermediates in electrophilic aromatic
substitution reactions,7 and a few stable arenium metal com-
plexes have been reported.1f,8,9 The first methylene-arenium
compounds were reported very recently.9 Difluoromethylene-
arenium complexes are unknown.
The design of well-defined soluble transition-metal complexes
capable of selective activation and functionalization of strong
single bonds under mild conditions is a highly desirable goal
of considerable current interest.1 Homogeneous activation of
strong, unstrained C-C and C-F bonds by the insertion of
platinum group metal complexes in solution is a major recent
topic.2-5 In continuation of our interest in C-C and C-F bond
activation,3,5 we now report on the consecutive C-C activation
and C-F cleavage in one system. The process involves selective
metal insertion into a very strong aryl-CF3 C-C bond, followed
(4) For some examples of intermolecular C-F activation by platinum
metals see: (a) Whittlesy, M. K.; Perutz, R. N.; Greener, B.; Moore, M. H.
Chem. Commun. 1997, 187. (b) Edelbach, B. L.; Jones, W. D. J. Am. Chem.
Soc. 1997, 119, 7734. (c) Hofmann, P.; Unfried, G. Chem. Ber. 1992, 125,
659. (d) Hughes, R. P.; Linder, D. C.; Rheingold, A. L.; Yap, G. P. A.
Organometallics 1996, 15, 5678. (e) Lopez, O.; Crespo, M.; Font-Bardia,
M.; Solans, X. Organometallics 1997, 16, 1233. (f) Burdeniuc, J.; Crabtree,
R. H. Organometallics 1998, 17, 1582. (g) Hintermann, S.; Pregosin, P.
S.; Ruegger, H.; Clark, H. C. J. Organomet. Chem. 1992, 435, 225. (h) For
a recent theoretical study on C-F bond activation see: Su, M.-D.; Chu,
S.-Y. J. Am. Chem. Soc. 1997, 119, 10178.
(5) (a) Blum, O.; Frolow, F.; Milstein, D. J. Chem. Soc., Chem. Commun.
1991, 258. (b) Aizenberg, M.; Milstein, D. Science 1994, 265, 359. (c)
Aizenberg, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 8674.
(6) (a) van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Vigalok, A.;
Milstein, D. Angew. Chem., Int. Ed. Engl. 1997, 36, 625. (b) van der Boom,
M. E.; Liou, S.-Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. J. Am.
Chem. Soc. 1998, 120, 6531.
* To whom correspondence should be addressed. Fax: +972-8-9344142.
E-mail: comilst@wiccmail.weizmann.ac.il.
(1) For some reviews regarding bond activation see: (a) Crabtree, R. H.
Chem. ReV. 1985, 85, 245. (b) Herrmann, W. A.; Cornils, B. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1048. (c) Saunders, G. C. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2615. (d) Burdeniuc, J.; Jedlicak, B.; Crabtree, R. H. Chem.
Ber./Recueil 1997, 130, 145. (e) Kiplinger, J. L.; Richmond, T. G.;
Osterberg, C. E. Chem. ReV. 1994, 373. (f) Steenwinkel, P.; Gossage, R.
A.; van Koten, G. Chem. Eur. J. 1998, 4, 759. (g) Arndtsen, B. A.; Bergman,
R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. (h)
Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. Engl.
1998, 37, 2180. (i) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed.
Engl. 1999, 38, 870.
(2) (a) Suzuki, H.; Takaya, Y.; Takemori, T. J. Am. Chem. Soc. 1994,
116, 10779. (b) Nicholls, J. C.; Spencer, J. L. Organometallics 1994, 13,
1781. (c) For a recent theoretical study on CsC bond activation see:
Holthausen, M. C.; Koch, W. J. Am. Chem. Soc. 1996, 118, 9932.
(3) (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature
1993, 364, 699. (b) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman, A.;
Ben-David, Y.; Milstein, D. Nature 1994, 370, 42. (c) Liou, S.-Y.; Gozin,
M.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 9774. (d) Liou, S.-Y.; Gozin,
M.; Milstein, D. J. Chem. Soc., Chem. Commun. 1995, 1965. (e) van der
Boom, M. E.; Kraatz, H.-B.; Ben-David, Y.; Milstein, D. Chem. Commun.
1996, 2167. (f) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D.
J. Am. Chem. Soc. 1996, 118, 12406. (g) Gandelman, M.; Vigalok, A.;
Shimon, L. J. W.; Milstein, D. Organometallics 1997, 16, 3981. (h) Liou,
S.-Y.; van der Boom, M. E.; Milstein, D. Chem. Commun. 1998, 687. (i)
van der Boom, M. E.; Ben-David, Y.; Milstein, D. Chem. Commun. 1998,
917. (j) van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Gozin, M.;
Milstein, D. J. Am. Chem. Soc. 1998, 120, 13415. (k) Rybtchinski, B.;
Milstein, D. J. Am. Chem. Soc. 1999, 121, 4528.
(7) Taylor, R. Electrophilic Aromatic Substitution; John Wiley & Sons:
New York, 1990.
(8) (a) van Koten, G.; Timmer, K.; Noltes, J. G.; Spek, A. L. J. Chem.
Soc., Chem. Commun. 1978, 250. (b) Grove, D. M.; van Koten, G.; Louwen,
J. N.; Noltes, J. G.; Spek, A. L.; Ubbels, H. J. C. J. Am. Chem. Soc. 1982,
104, 6609. (c) Grove, D. M.; van Koten, G.; Ubbels, H. J. C. Organome-
tallics 1982, 1, 1366. (d) Terheijden, J.; van Koten, G.; Vinke, I. C.; Spek,
A. L. J. Am. Chem. Soc. 1985, 107, 2891. (e) Steenwinkel, P.; Kooijman,
H.; Smeets, W. J. J.; Spek, A. L.; Grove, D. M.; van Koten, G.
Organometallics 1998, 17, 5411. (f) For a theorectical study on the formation
of [PtI(MeC6H3-(CH2NMe2)2-o,o′)]BF4] see: Ortiz, J. V.; Havlas, Z.;
Hoffmann, R. HelV. Chim. Acta 1984, 67, 1.
(9) (a) Vigalok, A.; Milstein, D. J. Am. Chem. Soc. 1997, 119, 7873. (b)
Vigalok, A.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 1998, 120,
477. (c) Vigalok, A.; Rybtchinski, B.; Ben-David, Y.; Shimon, L. J. W.;
Milstein, D. Organometallics 1999, 18, 895.
10.1021/ja990779g CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/02/1999

Formation of Difluoromethylene-Arenium Complexes
J. Am. Chem. Soc., Vol. 121, No. 28, 1999 6653
Scheme 1
difluorobenzyl cation.13 Prolonged standing of 4-I resulted in
some halide dissociation giving a complex formulated as
4-(dioxane)(OTf)2 (10-15%) as indicated by 1H, 31P{1H}, and
19F{1H} NMR and FD-MS of the reaction solution. The latter
shows a signal at M ) 667.1, suggesting the presence of [Rh-
(CF2)(2,6-(CH2PtBu2)2-C6H3)-(C4H4O2)]2+ in the reaction mix-
ture. Dicationic PCP type Rh(I) complexes have been reported
previously.9
Results and Discussion
Reaction of [RhClL2]2 (L ) C2H4 or C8H14) with 2 equiv of
1-CF3-2,6-(CH2PtBu2)2-C6H3 (1) in a toluene or dioxane solution
at 160 °C in a sealed pressure vessel resulted in oxidative
addition of the Ar-CF3 bond to Rh(I) yielding the complex
Rh(CF3)(2,6-(CH2PtBu2)2-C6H3)Cl (2-Cl) quantitatively. This
compound was unambiguously characterized by various NMR
techniques, FD-MS, and independent preparation of its iodide
analogue 2-I (Scheme 1). Complex 2-I was prepared by
oxidative addition of CF3I to the known Rh(I) complex Rh(η1-
N2)(2,6-(CH2PtBu2)2-C6H3) (3) in THF at room temperature.3i,j,10a,b
Interestingly, although one C-C and three benzylic C-F bonds
are available, only metal insertion into the unstrained sp2-sp3
C-C bond is observed.11 Products indicative of an ArCF2-F
bond cleavage process were not detected upon monitoring the
reaction by 31P{1H} and 19F{1H} NMR at room temperature.
Direct sp2-sp3 Ar-CH3, Ar-CH2CH3, and Ar-OCH3 oxidative
addition to Rh(I) was reported by us recently.3,6 In the Ar-C
oxidative addition, competitive C-H bond activation was
observed,3 whereas in the Ar-OCH3 bond activation processes
only Ar-O bond cleavage occurred with Rh(I).6 To our
knowledge, metal insertion into a strong, unstrained C-C single
bond of a fluorocarbon in solution is unprecedented. This unique
bond activation process may proceed via a concerted three-
centered transition state as recently elucidated for oxidative
addition of Rh(I) and Ir(I) to an Ar-CH3 bond.3f However, the
strongly electron withdrawing nature of the CF3 group may
make a nucleophilic attack on the arene by the electron-rich
metal center possible. It is noteworthy that Bergman et al.
observed C-C bond cleavage in hexafluoroacetone with (Me2-
PCH2CH2PMe2)2Ru(H)(OH) to afford (Me2PCH2CH2PMe2)2-
Ru(H)(OC(O)CF3) and CF3H.12
To probe the possibility of converting the Rh-CF3 group to
a carbonyl moiety,1e complex 2-Cl was reacted with excess
HOTf in a dioxane/H2O (95:5 v/v) solution at room temperature.
However, only 4-Cl was formed as judged by 19F{1H} and 31P-
{1H} NMR, which is stable in these acidic reaction conditions
for at least 24 h. The known Rh(CO)(2,6-(CH2PtBu2)2-C6H3)
was not observed by IR and 31P{1H} NMR analysis of the
product solution.1e,10c Noteworthy, treatment of benzylic PCP
type Rh(I) and Pt(II) complexes with HCl or HOTf resulted in
sp2-sp3 C-C bond activation.3c,e
Complexes 4-Cl,I were characterized spectroscopically by 1H,
31P, 19F, and 13C NMR, FD-MS, and UV/vis. Assignments in
1
the NMR spectra were made with use of H{31P} and 13C
DEPT135 NMR. Complexes 4-Cl,I exhibit almost identical
1
spectroscopic properties. The H NMR spectrum of the latter
t
shows two characteristic 1:2:1 triplets for the inequivalent Bu
groups at δ 1.01 and 1.13 (3JPH ) 5.9 and 7.1 Hz), respectively,
which collapse to singlets upon phosphorus decoupling. The
t
occurrence of the Bu triplet patterns testifies that the ligand
maintained a trans configuration.3f,i,j,5,9,10 The four protons of
the two diastereotopic CH2P groups appear as a typical AB
2
pattern at δ 2.77 (∆ABq ) 88 Hz, JHH ) 15.7 Hz). The two
meta protons appear as a doublet at δ 7.31 (3JHH ) 7.5 Hz) and
the para proton is observed at δ 8.24 as a double triplet (3JHH
5
) 7.5 Hz, JRhH ) 2.8 Hz), which is clearly not in the normal
region for aryl protons.8,9,12 The 31P{1H} NMR spectrum of 4-I
displayed a doublet of triplets at δ 25.1 (1JRhP ) 97.4 Hz, 3JPF
) 18.4 Hz) for the two magnetically equivalent trans phosphorus
atoms which are coupled to rhodium and to two fluoride nuclei.
The large ∆δ(4-2) ) - 38 ppm and the relatively small
rhodium-phosphorus coupling constants (1JRhP ∼ 100 Hz) are
indicative of the formation of methylene-arenium PCP-Rh(I)
complexes.9 The new CF2 moiety is clearly observed by 19F-
{1H} NMR as a double triplet at δ -44.9 (2JRhF ) 19.6 Hz,
3JPF ) 18.2 Hz), and is markedly shifted (∆δ >30 ppm) in
comparison to the signal of the Rh-CF3 group of the starting
compound. The FD-MS spectra of 4-Cl,I show the M+ - OTf
(581.1, 4-Cl; 673.1, 4-I) and a correct isotope pattern. Four new
distinctly different bands are seen in the UV/vis spectrum of
4-I at λ 600, 370 (sh), 320, and 260 nm, while no starting
material 2-I remained (λ 430 and 280 nm).14 Analogous
Remarkably, reaction of complexes 2-Cl,I with excess HOTf
(∼70 equiv) in dioxane or CH2Cl2 at room temperature resulted
in the immediate formation of the novel green arenium
complexes 4-Cl,I by an unusual organometallic sequence
involving C-F, M-C, and M-Ar bond cleavage, and C-C
bond formation. Moreover, this intriguing transformation also
involves a mild dearomatization process. No reaction was
observed with 2-Cl when HCl was used. The spectroscopic data
are fully in agreement with the formation of difluoromethylene-
arenium complexes (vide infra),9 the resonance form of a
(10) (a) [Ir(2,6-(CH2PtBu2)2-C6H3)]2(µ2-N2) was very recently structurally
characterized. Lee, D. W.; Kaska, W. C.; Jensen, C. M. Organometallics
1998, 17, 1. (b) Nemeh, S.; Jensen, C.; Binamira-Soriaga, E.; Kaska, W.
C. Organometallics 1983, 2, 1442. (c) Moulton, C. J.; Shaw, B. L. J. Chem.
Soc. 1976, 1020.
(13) (a) ArCF2+ species have been studied by Olah et al.: Olah, G. A.;
Mo, Y. K. J. Org. Chem. 1973, 38, 2686. (b) For C-F bond activation by
+
ArCF2 species see: Ferraris, D.; Cox, C.; Anand, R.; Lectka, T. J. Am.
Chem. Soc. 1997, 119, 4319.
(11) For a report on C-F versus C-C bond activation with lanthanide
cations in the gas phase see: Schwarz, H.; Hornung, G.; Cornehl, H. H. J.
Am. Chem. Soc. 1996, 118, 9960.
(12) (a) Kaplan, A. W.; Bergman, R. G. Organometallics 1997, 16, 1106.
(b) Kaplan, A. W.; Bergman, R. G. Organometallics 1998, 17, 5072.
(14) Arene carbonium ions generated in acidic media exhibit similar
spectroscopic properties; cyclohexadienyl cations exhibit an absorption band
in the UV/vis spectrum at λ ∼ 400 nm. Brouwer, D. M.; Mack, E. L.;
Maclean, L. In Carbonium Ions; Olah, G. A., Scheyer, P. V., Eds.; Wiley-
Interscience: New York, 1970; Vol. 11

6654 J. Am. Chem. Soc., Vol. 121, No. 28, 1999
Van der Boom et al.
Scheme 2
Scheme 3
difluorocarbene species and HF.1e,16 The aryl-M σ bond in PCP
type complexes is relatively inert in comparison to regular σ
aryl-M and alkyl-M bonds.17 However, insertion of terminal
acetylenes into the strong σ aryl-Ru(II) bond of a PCP complex,
which probably proceeds via intermediacy of unobserved
carbene species, was recently reported.18 Cleavage of aryl-Rh-
(I) and -Ru(II) σ bonds involving PCP ligand exchange3h,19
and insertion of a methylene group into the σ aryl-Rh(I) bond
of a PCP type complex upon reaction with MeI are also
known.3b
1
Table 1. Comparison of H NMR Data of Methylene 7-99 and
Difluoromethylene-Arenium PCP Complexes 4-Cl,I
δ, p-H
(ppm)
∆δ(complex - ligand)
complex
Μ
X
(ppm)
7
8
9
4-Cl
4-I
Ir
Cl
Cl
CO
Cl
I
9.20
8.60
7.98
8.31
8.24
2.4
1.8
1.18
1.11
1.04
Rh
Rh
Rh
Rh
methylene-arenium PCP-Rh(I) complexes have virtually
identical UV/vis spectra.9
As recently reported, the chloro(methyl) PCP metal(III)
complexes 5 and 6 react with HOTf affording the methylene-
arenium complexes 7 and 8 (Scheme 2),9b,c which have almost
identical spectroscopic properties as 4-Cl,I.
The dicationic carbonyl analogue [Rh(CH2)(2,6-(CH2PtBu2)2-
C6H3)(CO)]2+ (9) was obtained by treatment of 8 with AgOTf
and CO.9c NMR and X-ray analysis revealed that the positive
charge on the aromatic ring decreases in the order 9 < 8 < 7,
and it seems that the chemical shift δ and ∆δ(complex - free
ligand) of the p-H is a good probe for the extent of the positive
charge localized on the aromatic ring (Table 1). Therefore, we
may conclude that the amount of positive charge distributed in
the ring of complex 4-Cl,I is comparable to that of complex 9,
with the difluoromethylene-arenium resonance form being
dominant. Van Koten and co-workers reported a stable arenium
complex [PtI(MeC6H3-(CH2NMe2)2-o,o′)]BF4, which was ob-
tained by reaction of a cationic Pt(II) complex with MeI.1f,8 The
1H NMR spectrum of this interesting complex shows the para
and meta protons at δ 8.70 and 7.60 ppm, respectively.8b
In support of our proposed carbene mechanism involving
species such as B, reaction of complex 2-Cl with 1 equiv of
BF3‚OEt2 in CD2Cl2 resulted in the selective formation of [Rh-
(1-CF2-2,6-(CH2PtBu2)2-C6H3)Cl]BF4 (10) as judged by 1H, 31P,
and 19F NMR and UV/vis analysis of the green product solution
(Scheme 3). No starting material remained. The FAB-MS
spectrum of 10 shows the M+ - BF4 (581.1) and a correct
isotopic pattern. It is known that Lewis acids such as BF3 can
abstract fluorides from carbon to afford a MdCF2 species.1e,16,20
Moreover, complex 10 was prepared independently by reaction
of 2-Cl with 1 equiv of Ph3CBF4 overnight at room temperature
in CD2Cl2. Formation of Ph3CF was observed by 19F NMR and
GC-MS. The C-F bond cleavage process cannot be reversed
n
by reaction of 10 with Bu4NF.
Summary
As reported, it is likely that protonation of complexes 5 and
6 results in a 1,2-migration of the methyl group to the aromatic
ring, followed by â-H elimination to give complexes 7, 8, and
H2.9c Complex 5 undergoes a 1,2-methyl shift between the metal
center and the ipso carbon of the aromatic group upon treatment
with AgBF4 and CO in THF at room temperature.9b,c In contrast
to that, the CF3 group does not migrate upon reacting 2-I with
AgBF4 and CO under identical reaction conditions as indicated
by 31P{1H} and 19F{1H} NMR.15a Reductive elimination of an
alkyl group is more favorable than that of a trifluoromethyl
group.15b We do not believe that a pathway involving protona-
tion of the metal center and an unprecedented 1,2-migration of
the trifluoroalkyl group to the aromatic ring giving complex A,
followed by a rarely observed â-F elimination process, is
operative with 2-Cl,I.1e Although speculative, we suggest that
the formation of 4-Cl,I might proceed via an electrophilic attack
by H+ on the trifluoromethyl ligand, giving an unobserved Rhd
CF2 species B and HF, which is followed by a migratory
insertion of the aryl group to the difluorocarbene ligand. The R
C-F bonds of fluorinated ligands are activated toward elec-
trophiles due to π back-bonding of the metal center, and
therefore might react with strong acids to afford cationic
A number of unprecedented transformations have been
presented here. Selective oxidative addition of one of the
strongest C-C bonds, Ar-CF3, was demonstrated. This reaction
is selective and does not involve C-F activation. The RhIII-
(16) (a) For a review on formation and reactivity of transition-metal
dihalocarbene complexes see: Brothers, J. P.; Roper, W. R. Chem. ReV.
1988, 88, 1293. (b) Anderson, S.; Hill, A. F.; Clark, G. R. Organometallics
1992, 11, 1988. (c) Michelin, R. A.; Ros, R.; Guadalupi, G.; Bombieri, G.;
Benetollo, F.; Chapuis, G. Inorg, Chem. 1989, 28, 840. (d) Burch, R. R.;
Calabrese, J. C.; Ittel, S. D. Organometallics 1988, 7, 1642. (e) Richmond,
T. G.; Shriver, D. F. Organometallics 1983, 2, 1061. (f) Richmond, T. G.;
Shriver, D. F. Organometallics 1984, 3, 305. (g) Richmond, T. G.; Crespi,
A. M.; Shriver, D. F. Organometallics 1984, 3, 314. (h) Reger, D. L.; Dukes,
M. D. J. Organomet. Chem. 1978, 153, 67. (i) Clark, B. R.; Hoskins, S.
V.; Roper, W. R. J. Organomet. Chem. 1982, 234, C9.
(17) (a) Seligson, A. L.; Trogler, W. C. J. Am. Chem. Soc. 1992, 114,
7085. (b) Milstein, D. Unpublished results.
(18) (a) Jia, G.; Lee, H. M.; Xia, H. P.; Williams, I. D. Organometallics
1996, 15, 5453. (b) Lee, H. M.; Yao, J.; Jia, G. Organometallics 1997, 16,
3927.
(19) Dani, P.; Karlen, T.; Gossage, R. A.; Smeets, W. J. J.; Spek, A. L.;
van Koten, G. J. Am. Chem. Soc. 1997, 119, 11317.
(20) (a) For a report on the formation of a RudCF2 species by R C-F
migration see: Huang, D.; Caulton, K. G. J. Am. Chem. Soc. 1997, 119,
3185. (b) It is known that H+ or BF3 might act as F- scavengers by
formation of a relatively strong bond (B.D.E. H-F ) 135.9 kcal‚mol-1
,
(15) (a) King, R. B. Acc. Chem. Res. 1970, 3, 417. (b) Clark, H. C.;
Manzer, L. E. J. Organomet. Chem. 1973, 59, 411.
B-F ) 183 ( 3 kcal‚mol-1).1e,16 Handbook of Chemistry and Physics,
57th ed.; CRC Press: Boca Raton, FL, 1976-1977.

Formation of Difluoromethylene-Arenium Complexes
J. Am. Chem. Soc., Vol. 121, No. 28, 1999 6655
solid, followed by an aqueous solution of NaOAc (16 g, 50 mL). The
resulting solution was extracted twice with ether (2 × 250 mL). The
combined ether layers were dried over Na2SO4, filtered, and concen-
trated in vacuo. The residue was dissolved in pentane and filtered; white
crystals were obtained at - 30 °C (3.23 g, 76%). For 1: 1H NMR
CF3 complex reacts with electrophiles to produce the unique
difluoromethylene-arenium complexes. This unusual dearo-
matization process probably proceeds via a RhdCF2 intermedi-
ate. Interestingly, isostructural fluoro- and hydrocarbon meth-
ylene-arenium complexes were obtained, although the mechan-
ism involved is distinctly different.3f,9 Formation of these
arenium compounds seems surprising, since the electron-
withdrawing CF2 group is expected to destabilize such unusual
structures. However, it was shown that the CH2 group of
analogous methylene-arenium complexes 7-9 is hardly in-
volved in delocalization of the positive charge on the aromatic
ring.9 Stable arenium compounds are rare,1f,8,9 and moreover,
the difluoromethylene-arenium resonance form of a difluo-
robenzyl complex was thus far unknown. Interestingly, the
overall process involves cleavage of a sp3 C-F bond of a
fluorocarbon in solution promoted by prior metal insertion into
an Ar-CF3 C-C bond.
3
3
(CDCl3) δ 7.61 (d, 2H, JHH ) 7.8 Hz, ArH), 7.20 (t, 1H, JHH ) 7.8
Hz, ArH), 3.01 (s, 4H, CH2P), 1.02 (d, 36H, 3JPH ) 10.9 Hz, C(CH3)3).
5
31P{1H} NMR (CDCl3) δ 38.9 (q, JFP ) 7.3 Hz). 19F{1H} NMR
5
(CDCl3) δ - 49.8 (t, JPF ) 7.4 Hz, ArCF3). 13C{1H} NMR δ 141.30
(d, JCF ) 13.5 Hz, Ar), 130.41 (s, JCF ) 21.4 Hz, Ar), 129.89 (s, Ar),
127.1 (q, JCF ) 27.1 Hz,), 126.0 (q, JCF ) 277 Hz, Ar), 31.93 (d, JCF
) 22.2 Hz, C(CH3)3), 29.60 (d, JCF ) 13.3 Hz, C(CH3)3), 26.47 (dq,
JCF ) 2.7 Hz, ArCH2P). MS: 463 (M+ + 1).
Ar-CF3 C-C Bond Activation. Formation of 2-Cl,I. A toluene
or dioxane solution (10 mL) of 1 (24 mg; 0.040 mmol) and [RhCl-
(L)2]2 (L ) C2H4 or C8H14) (0.020 mmol) was loaded into a high-
pressure vessel and heated for 9 h at 160 °C. The resulting yellow
1
solution was analyzed by H{31P} and 19F{1H} NMR indicating the
quantitative formation of 2-Cl. Removal of the volatiles in vacuo
afforded complex 2-Cl as a yellow solid in quantitative yield. Traces
of ligand impurities and C8H14 can be removed by washing the residue
with cold pentane (-30 °C). The reaction can be run at lower
temperatures when a 5-fold excess of 1 is used, leading to the
quantitative formation of 2-Cl after heating at 120 °C overnight. No
other complexes were found. No C-C or C-F activation were indicated
by NMR upon reaction of 1 with other metal precursors such as [{IrCl-
(C8H14)2}2], (PhCN)2PtCl2, or Pd(CF3CO2)2. The iodide analogue of
2-Cl was obtained by reaction of CF3I with 3 under similar reaction
conditions as for the reaction of 3 with EtI.3j For 2-Cl: 1H NMR (C6D6)
Experimental Section
General Procedures. The procedures and spectroscopic analyses
are similar to those previously reported.3,6 CFCl3 was used as an external
reference at δ ) 0.0 in the 19F NMR. All reactions were carried under
an inert atmosphere in a nitrogen-filled drybox or by using standard
Schlenk techniques. Solvents were dried, distilled, and degassed before
use. Rh(η1-N2)(2,6-(CH2PtBu2)2-C6H3) and [RhClL2]2 (L ) C2H4 or
C8H14) were prepared according to published procedures.3i,j,21 BF3‚OEt2
n
was purchased from Merck. CF3I, Ph3CBF4, Bu4NF (1.0 M in THF),
2
HCl (4.0 M in dioxane), HOTF, and DOTf were purchased from Aldrich
and used as received. NMR spectra were recorded on a Bruker AMX
400 or an Bruker DPX 250 spectrometer. Ph3PO was used as an internal
standard for integration. FD-MS analyses were performed at the
University of Amsterdam.
δ 7.0 (m, 3H, ArH), 3.37 (dvt, 2H left part of ABq, JHH ) 17.0 Hz,
2
2JPH ) 4.2 Hz, CH2P), 2.82 (dvt, 2H right part of ABq, JHH ) 17.0
2
2
Hz, JPH ) 4.4 Hz, CH2P), 1.42 (vt, 18H, JPH ) 7.0 Hz, C(CH3)3),
1.05 (vt, 18H, JPH ) 6.0 Hz, C(CH3)3). 31P{1H} NMR (C6D6) δ 62.6
2
1
3
(dq, JRhP ) 116.7 Hz, JFP ) 16.3 Hz). 19F{1H} NMR (C6D6) δ 9.0
(dt, 2JRhF ) 21.3 Hz, 3JPF ) 16.5 Hz, RhCF3). FD-MS: M+ 600 (correct
isotope pattern). For 2-I: 1H NMR (C6D6) δ 7.0 (m, 3H, ArH), 3.42
(dvt, 2H left part of ABq, 2JHH ) 17.0 Hz, 2JPH ) 4.8 Hz, CH2P), 2.99
Formation of 1-CF3-2,6-(CH2PtBu2)2-C6H3 (1). Compound 1 was
prepared from 2-bromo-m-xylene by trifluoromethylation,22 bromina-
tion, and phosphination with di-tert-butylphosphine.10c (a) Preparation
of trifluoromethyl-m-xylene: A mixture of sodium trifluoracetate (27.2
g, 200 mmol), CuI (19.1 g, 100 mmol), N-methylpyrrolidone (400 mL),
and 2-bromo-m-xylene (9.25 g, 50 mmol) was stirred in a 1 L flask
and heated at 160 °C for 60 h. After the oil bath was cooled to 100 °C,
the reaction flask was connected to a vacuum system with a trap
immersed in liquid N2 to condense the product. The material in the
trap was collected with ether and washed once with an aqueous HCl
solution (10%) and twice with water. The ether layer was dried over
Na2SO4, filtered, and concentrated in vacuo. The residue was distilled
under water vacuum (bp 65 °C, yield 87%). 1H NMR (CDCl3) δ 7.09
2
2
(dvt, 2H right part of ABq, JHH ) 17.0 Hz, JPH ) 4.4 Hz, CH2P),
1.49 (vt, 18H, 2JPH ) 6.9 Hz, C(CH3)3), 1.03 (vt, 18H, 2JPH ) 6.1 Hz,
C(CH3)3). 31P{1H} NMR (C6D6) δ 62.4 (dq, JRhP ) 115.6 Hz, JFP
)
)
1
3
15.7 Hz). 19F{1H} NMR (C6D6) δ 10.9 (dt, JRhF ) 21.5 Hz, JPF
2
3
15.6 Hz, RhCF3). Anal. Calcd for C25H43I1F3P2Rh1: C, 43.37; H 6.26.
Found: C, 42.72; H, 6.01.
Reaction of 2-Cl,I with HOTf. Formation of Complexes 4-Cl,I.
A yellowish dioxane solution (1 mL) of complex 2-Cl,I (10 mg; 0.017,
0.015 mmol, respectively) was loaded into a 5 mm screwcap NMR
tube and treated with excess HOTf (0.1 mL, ∼70 equiv). The reaction
solution turned immediately green upon addition of the acid, indicative
of the formation of a methylene-arenium complex.9 19F{1H} and 31P-
{1H} NMR analysis after approximately 15 min indicated the quantita-
tive formation of 4-Cl,I (>95%), no starting material remaining.
Subsequently, the reaction solution was dried under high vacuum
yielding complex 4-Cl,I as a green powder. The reaction of 2-Cl with
HOTf proceeds also smoothly in the presence of H2O (50 µL), and no
formation of Rh(CO)(2,6-(CH2PtBu2)2-C6H3) was observed by IR and
31P{1H} NMR.10c No deuterium incorporation in 4-Cl,I was observed
when DOTf was used. No reaction was observed when HCl was used
instead of HOTf. Complexes 4-Cl,I have similar spectroscopic proper-
ties. 1H NMR of 4-Cl (CD2Cl2) δ 8.31 (t, 1H, 3JHH ) 7.6 Hz, p-ArH),
3
3
(t, 1H, JHH ) 7.6 Hz, p-ArH), 7.92 (d, 2H, JHH ) 7.6 Hz, m-ArH),
2.33 (q, 6H, JFH ) 3.3 Hz, CH3). 19F{1H} NMR (CDCl3) δ -53.0 (s,
CF3). 13C{1H} NMR δ 137.37 (q, JCF ) 2.1 Hz, Ar), 130.8 (s, Ar),
130.13 (s, Ar), 127.53 (q, JCF ) 28.4 Hz, Ar), 126.01 (q, JCF ) 276.28,
CF3), 21.38 (q, JCF ) 4.1 Hz, CH3). (b) Bromomethylation: A mixture
of trifluoromethyl-m-xylene (9.3 g, 53.3 mmol), NBS (19.2 g, 108
mmol), and AIBN (0.3 g) in CCl4 (150 mL) was refluxed for 2 h under
lamp light. The mixture was cooled, filtered, and concentrated in vacuo.
The product was purified by column chromatography (dry silica;
eluent: hexane/ether ) 95/5) and recrystallized from methanol at -
30 °C (yield 8.9 g, 50%, mp 64 °C). 1H NMR (CDCl3) δ 7.48 (m, 3H,
ArH), 4.63 (q, 4H, JFH ) 1.5 Hz, ArCH2Br). 19F{1H} NMR (CDCl3) δ
-52.7 (s, CF3). 13C{1H} NMR δ 137.4 (q, JCF ) 1.8 Hz, Ar), 133.36
3
7.40 (d, 2H, JHH ) 7.6 Hz, m-ArH), 3.18 (ABq, 4H, ∆ABq ) 140
Hz, 2JHH ) 16.9 Hz, CH2P), 1.46 (vt, 18H, 3JPH ) 7.1, C(CH3)3), 1.27
(s, Ar), 132.23 (s, Ar), 125.5 (q, JCF ) 30 Hz, Ar), 124.5 (q, JCF
)
3
(vt, 18H, JPH ) 7.1, C(CH3)3). 31P{1H} NMR of 4-Cl (dioxane-d8) δ
277, CF3), 30.02 (q, JCF ) 4.8 Hz, ArCH2Br). (c) Phosphination: An
acetone solution of di-tert-butylphosphine (3.25 g, 22.3 mmol, 20%
excess) was added to an acetone solution (25 mL) of R,R′-dibromo-
2-trifluoromethyl-m-xylene (3.06 g, 9.2 mmol) and the mixture was
refluxed for 2 h. Upon heating a white solid precipitated from the clear
reaction solution, which was collected and washed with degassed
pentane (2 × 250 mL). Degassed H2O (50 mL) was added to the white
1
3
23.5 (dt, JRhP ) 96.0 Hz, JPF ) 17.7 Hz). 19F{1H} NMR of 4-Cl
1
3
(dioxane-d8) δ -46.6 (vq, JRhP ) JPF ) 18.3 Hz, CF2; this signal is
1
3
observed as a dt in CD2Cl2; JRhP ) 19.3 Hz, JPF ) 18.3 Hz), -78.7
(s, O3SCF3). FD-MS of 4-Cl: M+ - OTf 581.1 (correct isotope pattern).
1H NMR of 4-I (C6D6) δ 8.24 (dt, 1H, 3JHH ) 7.5 Hz, 5JRhH ) 2.8 Hz,
p-ArH), 7.31 (d, 2H, 3JHH ) 7.5 Hz, m-ArH)), 2.77 (ABq, 4H, ∆ABq
) 88 Hz, 2JHH ) 15.7 Hz, CH2P), 1.13 (vt, 18H, 3JPH ) 5.9, C(CH3)3),
(21) van der Ent, A.; Onderdelinden, A. L. Inorg. Synth. 1973, 14, 92.
(22) Carr, G. E.; Chambers, R. D.; Holmes, T. F.; Parker, D. G. J. Chem.
Soc., Perkin Trans. 1 1988, 921.
1.01 (vt, 18H, JPH ) 7.1, C(CH3)3). 31P{1H} NMR of 4-I (C6D6) δ
3
25.1 (dt, 1JRhP ) 97.4 Hz, 3JPF ) 18.4 Hz). 19F{1H} NMR of 4-I (C6D6)

6656 J. Am. Chem. Soc., Vol. 121, No. 28, 1999
Van der Boom et al.
1
3
δ -44.9 (dt, JRhP ) 19.6 Hz, JPF ) 18.2 Hz, CF2), -78.7 (s, O3-
mg, 0.018 mmol) at room temperature overnight resulted also in the
SCF3). 13C{1H} NMR of 4-I (C6D6) δ 163.26 (vt,
J
) 6.7 Hz,
2+4
quantitative formation of 10. Formation of Ph3CF was observed by 19
F
PC
o-C), 146.8 (s, p-C), 134.03 (vt, 3+5J ) 10.0 Hz, m-C), 120.19 (q,
and GC-MS of the product solution. This reaction proceeds relatively
slowly: 55% conversion was observed after approximately 2 h at room
temperature by 19F and 31P NMR. Treatment of a green CD2Cl2 solution
PC
1JFC ) 317.6 Hz, O3SCF3), 38.65 (t, 1+3JPC ) 14.8 Hz, C(CH3)3), 36.39
(t, JPC ) 13.8 Hz, C(CH3)3), 30.96 (t, 2+4JPC ) 3.8 Hz, C(CH3)3), ≈30.0
2+4
n
(br, CH2P), 29.44 (t,
J
) 3.34 Hz, C(CH3)3). UV/vis of 4-I (0.03
PC
(1 mL) of complex 10 (10 mg, 0.017 mmol) with 1 equiv of Bu4NF
mM in CH2Cl2): λ (ꢀ) 600 (5600), 370 (sh, ∼33000), 320 (66000),
260 (75000). For comparison, UV/vis of 2-I (0.03 mM in CH2Cl2): λ
(ꢀ) 430 (27000), 280 (75000). FD-MS of 4-I: M+ - OTf 673.1 (correct
isotope pattern). Prolonged standing of the product solution of 4-I at
room temperature for 3 days resulted probably in formation of 4-(OTf)2
(10-15%). The compounds 4-I and 4-(OTf)2 were not separated.
(17 µL, 1 M solution in THF) at room temperature resulted in an
immediate color change to yellow. 19F and 31P{1H} NMR showed a
mixture of unknown composition. No starting material remained and
no formation of a Rh-CF3 species was indicated. Complex 10: 1H
(CDCl3) δ 8.41 (t, 3JHH ) 7.7 Hz), 7.41 (d, 2H, 3JHH ) 7.7 Hz, m-ArH),
2
3.24 (ABq, 4H, ∆ABq ) 142 Hz, JHH ) 17.2 Hz, CH2P), 1.47 (vt,
3
Selected spectroscopic data: 1H NMR of 4-(OTf)2 8.05 (dt, 1H, JHH
18H, 3JPH ) 7.6 Hz, C(CH3)3), 1.29 (vt, 18H, 3JPH ) 7.3 Hz, C(CH3)3).
5
) 7.4 Hz, JRhH ≈ 3.0 Hz, p-ArH). 31P{1H} NMR of 4-(OTf)2 (C6D6)
1
3
31P{1H} (CDCl3) δ 25.77 (dt, JRhP ) 97.7 Hz, JPF ) 18.1 Hz). 19F-
δ 21.5 (dt, 1JRhP ) 96.2 Hz, 3JPF ) 19.1 Hz). 19F{1H} NMR of 4-(OTf)2
{1H} (CDCl3) δ -44.9 (dt, JRhP ) 19.6 Hz, JPF ) 16.3 Hz), 149.9
(br, BF4-). UV/vis (CDCl3): λ 585, 355, 309, 275. Anal. Calcd for
C25H43Cl1B1F6P2Rh1‚CHCl3: C, 39.63; H 5.63. Found: C, 40.24; H,
1
3
1
(C6D6) δ -47.9 (vq, JRhP
)
3JPF ) 20.4 Hz, CF2). FD-MS of
4-(OTf)2: [M + (dioxane) - (OTf)2] 667.1 (correct isotope pattern).
Reaction of 2-Cl with BF3 and Ph3CBF4. Formation of Complex
10. A cold yellowish CD2Cl2 solution (1 mL, - 30 °C) of complex
2-Cl (10 mg, 0.017 mmol) was treated with 1 equiv of BF3 in ether
using a microsyringe and was loaded into a 5 mm screwcap NMR tube.
An excess of BF3 (10 equiv) can be used as well. The reaction solution
5.57. FAB-MS: M+ - BF4 581.1 (correct isotope pattern).
-
Acknowledgment. This research was supported by the
U.S.-Israel Binational Science Foundation, Jerusalem, Israel,
and by the MINERVA foundation, Munich, Germany. D.M. is
the holder of the Israel Matz professorial chair of organic
chemistry. We thank Han Peeters (University of Amsterdam,
The Netherlands) for performing the FD-MS experiments.
1
1
was left at room temperature and turned green within 5 min. H, H-
{31P}, 31P{1H}, and 19F{1H} analysis after 1 h showed disappearance
of the starting material and the selective formation of complex 10.
Removal of the volatiles in vacuo afforded complex 10 as a green solid
in quantitative yield. Treatment of a yellowish CD2Cl2 solution (1 mL)
of complex 2-Cl (10 mg, 0.017 mmol) with 1 equiv of Ph3CBF4 (6
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